1. Technical Field of the Invention
The invention relates generally to hard disk drives (HDDs); and, more particularly, it relates to timing recovery that is performed within such HDDs.
2. Description of Related Art
As is known, many varieties of memory storage devices (e.g. disk drives), such as magnetic disk drives are used to provide data storage for a host device, either directly, or through a network such as a storage area network (SAN) or network attached storage (NAS). Typical host devices include stand alone computer systems such as a desktop or laptop computer, enterprise storage devices such as servers, storage arrays such as a redundant array of independent disks (RAID) arrays, storage routers, storage switches and storage directors, and other consumer devices such as video game systems and digital video recorders. These devices can be found in a wide variety of applications and can also employ other types of means by which to store information (e.g., optical storage systems). These devices, which also can include any type of rotating storage means, provide high storage capacity in a cost effective manner.
Within such memory storage devices, there is oftentimes a difficulty in performing timing recovery of a signal that is read from the disk, in that, appropriate digital sampling needs to be performed for accurate and effective recovery of the information within the signal read from the disk. For example, if improper digital sampling is performed on the signal that is read from the disk, then some of the data can potentially be inaccurately recovered. In another scenario, large portions or even all of the information within the signal read from the disk will not be able to be recovered. When this occurs, the system may try to re-read the data (e.g., via a re-try) which can be very time consumptive. In devices employing HDDs in which energy is at a premium (e.g., small hand held devices and/or battery operated devices), performing more disk accesses and read attempts than necessary is very wasteful in terms of the overall energy budget and will clearly reduce the time during which the device can be operated on a limited energy source.
Another source of potential problems with respect to timing recovery within devices employing HDDs is when the disk within the HDD is somehow off-center in its mounting. More specifically, a disk is mounted and turns so that the read/write heads can effectively access various portions of the disk surface or surfaces. Tracks of data are ideally circular in shape, and if mounted perfectly in the center of the disk, then the tracks of the disk are perceived as being without variation (i.e., they are smooth and the amplitude of the signal associated with the tracks does not vary as a function of location on the disk). However, through a variety of undesirable events that result in the imperfect central mounting of the disk within the device (e.g., improper central mounting of the disk during fabrication, mis-alignment of the disk's mounting resulting from an impact of the device such as if the device is dropped, etc.), the frequency of the signal associated with the tracks will then in fact vary as a function of location on the disk. The track following servo of the device must track radial run out and keep the amplitude of the signal near a constant level. For example, without this capability, the runout of approximately 1/1000 of an inch could amount to approximately 100 tracks of the device being in error. With the RPM of the media being constant, the linear velocity of the head is proportional to the actual radius, and this radius clearly varies of the disk within the device is not properly centrally mounted. Since the frequency of the signal is proportional to the linear velocity, the frequency undergoes modulation with once around variation when the disk within the device is not properly centrally mounted.
Therefore, this variation of the frequency of the signal associated with the tracks can be viewed as being a modulation error. In such instances, the tracks of the disk actually look like an ellipse (and not a circle) when viewd from the center of rotation. Within devices employing disks having smaller form factor, the overall percentage of modulation error can be significantly bigger if the mounting of the disk is off-center. This modulation error looks like a repeatable frequency error when trying to read information from the tracks of the disk.
It is always desirable to perform sampling of a signal read from a disk within a HDD at the appropriate time, but when these problems as described above, among others, are existent within such a device, then improper sampling of the signal read from the disk can be catastrophic in terms of the devices performance. For example, the data stored within the HDD may simply be non-recoverable in the event that sampling is performed at the improper time.
For accurate performance and proper recovery of the data within the signals read from disk, the precision required for sampling can be seemingly extreme. For example, control of sampling frequency to parts per million is sometimes required of beneficial. There clearly exists a need in the art for a means by which timing recovery can be performed efficiently and with a high degree of precision.